Electric lamp



S. RUBEN ELECTRIC LAMP Aug. 23, 1938.

Filed Dec. 20, 1955 INVENTOR vSAMUEL RUBEN Patented Aug. 23, 1938 PATENT OFFICE ELECTRIC LAllIP Samuel Ruben, New Rochelle, N. Y. Application December 20, 1935, Serial No. 55,344

, Claims. (c1. ire-1) This invention relates to an electric lamp and particularly to a lamp operating by means of the electron bombardment of a refractory body in a low pressure atmosphere.

Dne of the objects of this invention is the provision of an incandescent lamp of high efficiency, one that has a long life and one that can be commercially produced with a satisfactory degree of uniformity at low cost.

Other objects of the invention will be apparent from the description.

Broadly, the device of my invention consists of a lamp in which a refractory metal anode is incandesced by electron bombardment from a high electron emission cathode in a gaseous atmosphere continuously at a pressure low enough to cause a restricted ionization and having a limited degree of rise with increase in temperature.

Lamps of this general type of the prior art have several limitations such as arcing, a localized discharge and the gradual absorption of the ionization gases. In those of the pure electron emission types the space charge between the cathode and anode limits the current flow to insuillcient values.

I have found that by having a cathode and an anode having specific properties, within an atmosphere sufilcient in pressure to reduce'the electron space charge but insumcient to allow an arc discharge, many of those limitations heretofore considered inherent, are eliminated. The cathode is preferably of the indirectly heated type, one having a high emission surface, such as a coating of strontium and barium oxides. The anode is a dislr or plate made from a material having a high melting point and a low vapor pressure, such as boron carbide; the ionizing atmosphere has a pressure sufilcient to reduce the electron space charge to permit a non-critical positioning of the electrodes, the pressure charge also being insufficient to allow a large preponderance of ionic conductlon over the electron conduction or permit an arc discharge through it. The ionizing medium has a low enough pressure under all operating conditions to permit its use in an electron bombardment type of lamp. I have found that the best results are obtained in such a medium from a source consisting of an alloy of mercury and cadmium having a pressure less than .001 mm. Hg at 20 C. A fundamental requirement for the successful operation of the device is that the electron emission discharge capacity 01 the cathode be greater than the re quired bombardment currents; otherwise, de-

JstructiVe effects upon the emission coating fol- For the conductive refractory anode l have used such materials as tungsten, tantalum, tantalum carbide, various zirconium compounds 5 and baron carbide. Boron carbide can be pressed and sintered, but the anode need not be of any particular form; this electrode can be maintained at a high temperature without excessive blackening by evaporation and without sputtering upon the bulb because of the high melting point and low vapor pressure of this material. The anode is placed opposite one end of the cathode where it is best as a source of illumination, there being a minimum of shadow cast. Were it not for the ionization of the vapor atmosphere, the space charge would be so high as to preclude this positioning of the anode and the current would be limited to a very low value. The atmospheric pressure is insufficient to allow runaway characteristics due to large ionic conduction currents.

The metal vapor source must be one that will remain substantially wholly within the area in which it is initially deposited.

I have employed various alloys and compounds of mercury for the ionizable vapor source, such as those of sodium, bismuth and potassium, all of which I have found to possess merit; but I prefer that of cadmium because its chemical and physical properties are more satisfactory and make for greater ease of handling, the amalgam also being more stable.

The particular choice oi the cadmium-mercury alloy used is somewhat dependent upon the operating conditions, such as a maximum bulb operating temperature. The following table indicates the reduction of pressure by the combination of cadmium with mercury. The fractional vapor pressure of mercury (P/Po when P is the pressure over the amalgam Po the vapor pressure of Hg at the same temperature) is in equilibrium with the amalgam containing the mole fraction X. at 322 C.:

'P/Po' .891 .765 .630 .487 .340 .213 .160 .085 .012

then sealed into a short piece of evacuated glass tubing along with a few granules of magnesium powder which absorbs the undesirable gases. The glass tube in turn is inserted into a nickel cylinder and placed in the bulb. After the entire bulb is evacuated and the cathodes have been activated, the nickel cylinder and its contents are inductively heated, thus causing the wire to expand and blow out the glass container which is very soft and fluid at this temperature. The volatilized compound condenses on the glass wall adjacent to the pill.

The important advantage of mercury-cadmium compound is its constant position in the glass tube in relation to the other elements and the practicability of uniform duplication allowing an ionic device to be made within close limitations of operating vapor pressures. While the pressure in the device is only fractionally that of mercury, from one-third to one one-hundredth, under operating conditions it is adequate to allow a reduced space charge, but insuflicient to maintain a localizedor arc condition.

Mercury alone, due to its relatively high vapor pressure, would cause excessive ionic conduction and not allow the maintenance of a potential drop greater than its ionization potential which factor is necessary in order to have adequate power dissipation on the anode.

For a better understanding of the invention reference is made to the drawing in which I is a nickel cathode of the indirectly heated type having an electron emission coating 2, composed of strontium and barium oxides. At 3 is a tungsten coil heater having an insulating coating of aluminum oxide, produced as described in my United States Letters Patent Number 1,926,407, 4 being a lower support and conductor element for tubular cathode. At 5 is a cap that fits over the top of the cathode having a central hole through which passes magnesium oxide cylinder 6, surrounding a tungsten conductor rod I, and to which is fastened a disk anode of boron carbide 8. The cathode is supported by metal rod 9, mounted upon which is a nickel cylinder l0, containing a short glass tube which has within it mercurycadmium wire. The glass container is at H.

In operation the cathode is heated by the insulated tungsten coil and when electron emission temperature is reached electrons bombard the boron carbide anode to an incandescent temperature.

Having. described my invention, what I claim as new and desire to secure by Letters Patent, is:

1. An electric lamp comprising an indirectly heated cathode and an incandescible anode consisting of a plate of a refractory metallic carbide opposite an end of the cathode and crossing the axis of the cathode, in an ionizing medium from a source consisting of an alloy of mercury and cadmium and having a pressure less than .001 mm. Hg at C.

2. An electric lamp comprising an indirectly heated cathode, an anode composed of a boroncarbide compound, heated to incandescence by an electron bombardment from said cathode which has an electron emission capacity above that required to product anode incandescence, in an ionizing medium, the source of which consists of an amalgam of mercury and cadmium in which the mole fraction of mercury to cadmium is less than 0.5.

3. An electric lamp comprising an indirectly heated cathode, an anode consisting of a metallic carbide plate located beyond an end of and substantially at right angle to said cathode, heated to incandescence by an electron bombardment from the cathode, in an ionizing medium having a pressure slightly less than .001 mm. Hg at 20 C.

4. An electric lamp comprising an indirectly heated cathode, an anode of refractory incandescible material heated to incandescence by an electron bombardment from said cathode which has an electron emission capacity above that required to produce anode incandescence, an ionizing medium comprising a composition of mercury and another metal in which the mole fraction of mercury to said other metal is less than 0.5.

5. An electric lamp comprising an indirectly heated cathode, an anode of refractory incandescible material, heated to incandescence by an electron bombardment from said cathode which has an electron emission capacity above that required to produce anode incandescence, an ionizing medium comprising an amalgam of mercury and cadimum in which the mole fraction of mercury to cadmium is less than 0.5.

SAMUEL RUBEN. 

